Hard disk drives (HDD) contain magnetic transducers that magnetize recording media and sense the magnetic field of a rotating disk. The hard disk drive design must constantly evolve to meet the increasing demand of computer applications and configurations. One component of the HDD design that has had to evolve is the spindle motor bearing. The spindle is a rod-like axle inside a HDD. The disks inside the HDD are center mounted on the spindle and the spindle and the spindle motor rotates the spindle and the disks. In the past, the majority of HDDs implemented ball bearing spindle motors. However, the HDD industry is now using a different type of bearing design known as Fluid Dynamic Bearing (FDB).
FDB spindles are used because FDB spindles have significantly less runout, larger damping and lower acoustic noise. As head-disk separation diminishes and the sensitivity of recording transducers increases, the effects of spurious charge buildup caused by friction becomes an increasing concern. In the conventional HDD illustrated in FIG. 1A, the head to disk electrical breakdown due to inadequate grounding of the disk rotation leads to electrical noise on the disk coupling to the reader thereby increasing the bit error rate. In the illustration in FIG. 1A, the thermal fly-height control (TFC) of the HDD is turned off and an electrical breakdown is more likely to occur between the disk and the read sensor thereby damaging the reader. In the example shown in FIG. 1B, the TFC is turned on leading to an electrical breakdown as more likely to occur between the reader/shield and the disk. The presence of high-frequency noise transients of the disk can also degrade the bit error rate performance of the HDD.
There are several mechanisms that raise the electrical potential of the disk relative to the head. First, induced voltage from the spindle motor winding, tribocharge from the fluid dynamic bearing oil, and/or the tribocharging of the spindle due to internal rubbing on the motor.
In disk drives, the tribocharging system is complicated and involves multilayers of metal and insulators, a thin lubricant layer and complex contact surfaces. Tribocharging of the interface between the head and disk contributes to a voltage build up on the head when the disk is accelerating or decelerating. In order to solve the tribocharging and other electrostatic problems, special additives are added to the fluid dynamic bearing oil. However, this affects the performance of the fluid dynamic bearing and generally not sufficient. Even “conductive” oils are still relatively high in resistance and therefore not a viable solution because of cost and excessive resistance.
Thus, a better way of electrically grounding the fluid dynamic bearing to minimize the head to disk electrical breakdown and to provide adequate grounding to the fluid dynamic bearing is desired.